1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a shaft seal for a reactor coolant pump which utilizes shape memory metal.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2250 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft and their combined purpose is to provide for minimal reactor coolant leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U.S. Pat. Nos. to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222) and Boes (4,275,891) and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
The pump shaft sealing assemblies, which mechanically seal the interface between the stationary pump pressure boundary and the rotating shaft, must be capable of containing the high system pressure (approximately 2250 psi) without excessive leakage. The tandem arrangement of three sealing assemblies is used to break down the pressure in stages. These three mechanical pump sealing assemblies are controlled-leakage seals which in operation allow a minimal amount of controlled leakage at each stage while preventing excessive leakage of reactor coolant from the primary coolant system to respective seal leakoff ports.
The pump sealing assemblies are normally maintained at temperatures well below those of the primary coolant system, either through injection of cool fluid at the sealing assemblies or through use of a heat exchanger which cools the primary fluid before it reaches the sealing assemblies. Theorized failure of these systems may expose the sealing assemblies to high temperatures which, in turn, could lead to failure of the sealing assemblies. Failure of the sealing assemblies for other reasons may result in leakage beyond the capacities of the injection and the cooling systems. In both cases the sealing assemblies are exposed to high temperatures. One initiates sealing assembly failure. The other results from sealing assembly failure. In any event, total catastrophic failure of the sealing assemblies hypothetically could lead to excessive leakage rates, which at extremes might lead to reactor coolant uncovering of the reactor core and subsequent core damage.
Consequently, a need exists for an effective way to back up the standard sealing assemblies in the event of their failure so as to prevent excessive leakage therethrough.